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Thread: Dialarc 250 HF High Frequency Transformer

  1. #1
    Join Date
    Mar 2001
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    Saranac Lake, NY
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    Question Dialarc 250 HF High Frequency Transformer

    My new-to-me Dialarc 250 HF has weak HF -- I have to get the electrode very close to the work for the arc to start.

    The tungsten points in the HF unit are pristine and correctly gapped. The high voltage capacitor (C2) shows infinite resistance on my multimeter.

    Miller tech support told me that the transformer (T2) secondary should measure 6100 ohms, and if it doesn't, it should be replaced. Mine measures 1250 ohms.

    All well and good, but a new transformer isn't cheap, and it's going to be a PITA to install -- have to take apart half of the welder to do it -- so I thought I would ask here before I start digging. Anyone know anything about this transformer? I've seen Macona's name before on posts about Dialarc 250s. Is there anything else to test before doing a transformer transplant? What could cause the primary resistance to fall from nominal 6100 to 1250? There is no visible sign of overheating.

    Any new information on whether to replace and/or how to install will be appreciated.

  2. #2

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    I am not an expert on welding machines, but I have spent time on my Magna Tran Eutectic machine. It is very similar inside to your Dialarc in that it is a mag amp based unit with a spark gap HF generator.

    There isn't much you can do to your HF trans without removing it. Mine has a resistor in the spark circuit to vary spark intensity. Check if yours has one, is clean tight etc. I would replace the capacitor. The spark energy is coupled into the torch using a (likely) air core trans. Check to see if it is clean, turns not touching, windings not touching each other. Check torch lead for damage that could cause the HF to bleed off.

    If none of that helps I'd pull the HF trans and assuming it is an open core/coil construction I would carefully examine the assm for broken wires, burns. A low resistance reading would indicate that there is a failure path across a large portion of the coil. Dead ???? crawled into open ends of coil? HV coil is going to be fine wire, check for broken strands, crappy solder joints, etc.

    If you can't find something it's either replace or rewind. Replace is easy. Rewind is a PITA and will likely cost as much as a replmt trans. The trans is likely Class A, paper and thermoset varnish good for 105*C.

    If you replace the cap the Internet is your friend. Same or higher voltage rating, similar ufd rating. Cleanliness is important. Check for carbon tracks. DO NOT use brake cleaner as it may degrade wire coating and varnish. Use made for elec cleaner.

    Good Luck

  3. #3
    Join Date
    Jan 2004
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    Missouri
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    If the info on resistance is good, then you seem to have a short across a good bit of the coil, or the whole coil.

    That loses energy two ways.....

    one, it may reduce voltage by using fewer turns of wire, if the coil is partly shorted internally

    two, whatever is lowering the resistance is drawing power away from the HF arc.

    To measure, though, you need to have the transformer disconnected from anything that "goes across" (is in parallel with) the secondary. If it STILL measures low when isolated, it probably is the problem, but still could be dirty. BUT, if there is anything else, as in dirt, debris, etc, then when the coil is isolated, it may measure good, showing that the problem is somewhere else.

    There are circuit cleaners made for electronic cleaning. They can be used to "hose off" dirt and so forth, without damaging components. McMaster has them, as well as Digi-Key, etc. Other solvents like brake cleaner, may dissolve portions of components, or remove markings.

    There is a tendency for welding and grinding to make conductive dust. That can get deposited inside the unit due to air pulled through by fan cooling, and may cause shorts, low resistance, etc. Make sure that is not the issue before replacing expensive stuff.
    1601

    Keep eye on ball.
    Hashim Khan

  4. #4
    Join Date
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    Anderson SC
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    The transformer may well be the problem BUT its far more common for the capacitors to go bad on old HF start welders. I have fixed a couple that had very weak HF by replacing the caps, it made a huge difference. The caps are high voltage doorknob type, they are on ebay frequently. The HF circuit is a RF oscillator and the caps are critical. Next, I believe the dial arc has a set of arc gap contacts that must be clean and gapped properly, those are a routine maintenance item. A bad transformer would be rare, but possible.

  5. #5

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    I would agree that a cap is the most likely. The tesla coil crowd spends a lot of time with similar circuits. Have a look at the rating of the xformer. An oil furnace spark transformer is probably similar and used ones are available for free. They put out 23 ma at 10,000 volts. If you only need 5000, the furnace transformer is center tapped so you can use either output pole to ground. A common failure mode for both OBIT (Oil Burner Ignition Transformer) or a neon sign transformer is that they are potted in tar and the tar dries out allowing arcing which builds up a Carbon trace and lowers the resistance. A lot of TC types will rescue a transformer by melting the tar and letting it re solidify.

    Brian

  6. #6
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    Quote Originally Posted by bhowden View Post
    I would agree that a cap is the most likely. The tesla coil crowd spends a lot of time with similar circuits. ....... A lot of TC types will rescue a transformer by melting the tar and letting it re solidify.

    Brian
    Hah.... I have a similar transformer that will not get above 7000V, but is supposed to do 15000. I had not thought about the tar issue, but it is indeed "potted" in tar.

    Do you know what sort of temp is generally used? I could experiment, but knowledge beats "experience" (the kind from bad judgement).
    1601

    Keep eye on ball.
    Hashim Khan

  7. #7

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    I have not actually done it myself so this is second hand knowledge..... A quick google will turn up lots of people who have. The general advice is to put it in an oven at 375 - 400F. When the tar starts to soften, reduce to 300 and cook until liquid. Probably better to not use your wife's oven if you want to avoid learning some new good judgement. You can also dissolve the tar with kerosene or diesel fuel. Some people remove the tar and then submerse the coil in dielectric oil but this leaves you with a messy thing to contain. I have heard of people that just plug the transformer in and short the secondary until it generates it's own heat to melt the tar but this strikes me as a bit hit and miss. Personally I would check the amperage and if you are in the ball park, substitute an OBIT. They are dirt cheap and very reliable.

    Brian

  8. #8
    Join Date
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    Bad caps, they may test ok with an ohm meter but under HV, not a chance. These things have a limited life and are not cheap either.

    Used to be a miller service tech, i have replaced a lot of these capacitors. Rarely replaced any of the transformers.

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